A gas sensor such as an oxygen sensor, a NOx sensor or an HC sensor is known in the prior art. This gas sensor is generally provided with a protector for covering and protecting such a leading end portion of a gas sensor element as will be exposed to the object gas such as an exhaust gas. The gas sensor having such protector is disclosed in JP-A-2000-171429, JP-A-2002-162377 and JP-A-2000-304719, for example.
In the gas sensor disclosed in JP-A-2000-171429, an element cover (or a protector) has a double tube structure, in which bottomed cylindrical outer and inner tubes are overlap. Side holes for passing the object gas are individually formed in the side portion of the outer tube and in the side portion of the inner tube. The side holes in the outer tube are arranged on the trailing end side, and the side holes of the inner tube are arranged closer to the leading end side than the side holes of the outer tube. Moreover, bottom holes for passing the object gas are also formed in the bottom portion of the outer tube and in the bottom portion of the inner tube.
In the gas sensor disclosed in JP-A-2002-162377, a protective tube (or a protector) is composed of an outer protective tube and an inner protective tube individually having air vent holes. The air vent hole of the outer protective tube is formed in the side wall of the outer protective tube. The air vent hole of the inner protective tube is formed in a flange portion disposed on the trailing end side of the inner protective tube, and this flange portion is arranged in a through hole of a metal shell. Moreover, air vent holes are also formed in the bottom portions of the outer protective tube and the inner protective tube.
In the gas sensor disclosed in JP-A-2000-304719, a protective cover (or a protector) is provided with an inner protective cover covering the leading end portion of a sensor element, an outer protective cover covering the inner protective cover, and an intermediate protective cover interposed between those covers, thereby to construct a triple structure. An outer gas introducing hole of the outer protective cover is formed in the leading end side of the outer protective cover. An intermediate gas introducing hole of the intermediate protective cover is formed in a flange portion, which is disposed at the back of the cylindrical portion and abuts against the inner wall of the outer protective cover. An inner gas introducing hole of the inner protective cover is interposed, as viewed in the axial direction, between the outer gas introducing hole and the inner gas introducing hole. Moreover, an inner gas discharge port is formed in the bottom portion of the inner protective cover.